1. Field of the Invention
This invention relates to an optical modulator, and more particularly to a semiconductor optical modulator and a method of producing the semiconductor optical modulator.
2. Description of the Invention
Very high speed, large capacity transmission and information processing equipment optical communications technology is being developed rapidly, and in optical communications, a semiconductor laser output is modulated to effect transmission.
However, when a semiconductor laser is modulated directly in an optical transmission in the 1.5 .mu.m band at a rate higher than 1 Gbit/sec, a wavelength shift of the refraction index, that is, so-called wavelength chirping, is caused by variation in the refraction index arising from variation in carrier concentration in an active layer, restricting the communication distance. Much attention has been paid to the problem, and much effort directed to realizing an external modulator as a countermeasure to reduce the influence of wavelength chirping.
One such optical modulator is a bulk structure modulator of the electric field absorbing type which employs a bulk semiconductor for a light absorbing layer and makes use of an absorbing end variation by a Franz-Keldysh (F-K) effect as disclosed, for Example, in the Lecture No. C-152, Spring Meeting of the Electronic Information Communication Society of Japan, 1993 or Japanese Patent Laid-Open Application No. Heisei 4-303982. The optical modulator has a ridge type embedding structure, produced using the MOVPE (Metal Organic Vapor Phase Epitaxial) selective growth technology and does not involve etching of a semiconductor. Accordingly, the optical modulator can be anticipated as an optical modulator having a high degree of uniformity and reproducibility. The optical modulator is produced by first forming a pair of SiO.sub.2 stripe masks 52 having opening area 53 therebetween on n-type InP substrate 51 as shown in FIG. 1a, selectively growing n-type InP clad layer 54, InGaAsP light absorbing layer 55 and p-type InP clad layer 56 in opening area 53 as shown in FIG. 1b by the MOVPE method, then expanding opening area 53 as shown in FIG. 1c and finally growing p-type InP embedded layer 57 (carrier concentration: 5.times.10.sup.17 cm.sup.3) and p-type InGaAs cap layer 58 as shown in FIG. 1d. The optical modulator operates so that, when a reverse bias voltage is applied, a light absorption curve in the proximity of a fundamental absorption end of the light absorbing layer is shifted to the longer wavelength side (lower energy side) by the F-K effect to extinguish the layer light.
In order to improve the extinction characteristic of the optical modulator, the intensity of the electric field applied to light absorbing layer 55 must be increased to increase the wavelength shift amount. To this end, in the conventional optical modulator having the structure described above, the carrier intensities of n-type InP clad layer 54 and p-type InP clad layer 56 between which light absorbing layer 55 is held are set high (carrier concentration: 5.times.10.sup.17 cm.sup.-3) to increase the electric field intensity. Meanwhile, in order to reduce the capacitance of the device, the carrier concentration of n-type InP substrate 51 is set to 2.times.10.sup.17 cm.sup.-3. Consequently, a concentration difference appears at a growth interface between substrate 51 and n-type InP clad layer 54. In this instance, if a reverse bias voltage is applied to the device, the carrier concentration exhibits a difference across the growth interface and the extent of a depletion layer is different across the growth interface and creates a discontinuous portion as seen in FIG. 2. In the conventional optical modulator, depletion layer width r of n-type InP clad layer 54 is 640 angstrom at a carrier concentration of 5.times.10.sup.17 cm.sup.-3 while depletion layer width R of n-type InP substrate 51 is 1,210 angstrom at another carrier concentration of 2.times.10.sup.17 cm.sup.-3, and a difference of about 600 angstrom appears between them. Accordingly, the conventional optical modulator has a problem in that an edge breakdown occurs at a portion where a depletion layer exhibits a change in width, and the edge breakdown degrades the voltage resistance of the device.